There are enough of ready-to-use utilities and libraries, such as Open Source and multi-platform ImageMagic. Please see:
http://en.wikipedia.org/wiki/ImageMagick[
^],
http://www.imagemagick.org/[
^].
As it comes with command-line utilities, you can easy write a batch file of one or two lines, ordering something like: "convert all files in working directory", or the same in specified directory, or the same in working directory and all sub-directories, recursively, of "from the specified file list", or anything like that.
The required ability to write batch files is rudimentary for an advanced user, not even a software developer, you can read about it in any elementary manual.
Writing your own C# code is also pretty easy:
http://stackoverflow.com/questions/41665/bmp-to-jpg-png-in-c-sharp[
^].
—SA